This invention relates generally to ocular surgical procedures and instruments, and, more specifically, to a method and apparatus for measuring the visual acuity of the human eye, utilizing patient responses to visual prompts, during and immediately after ocular surgery. The invention is particularly useful in the field of corneal refractive surgery.
The cornea is the transparent tissue constituting the anterior sixth of the outer wall of the eye; its normal radius of curvature being 7.8 mm. Deviations from the normal shape of the corneal surface can produce errors of refraction in the visual process if the remainder of the eye's dimensions do not scale accordingly. The emmetropic eye, in a state of rest, focuses the image of distant objects exactly on the retina. Such an eye enjoys distinct vision for distant objects without effort. Any variation from this standard constitutes ametropia, a condition in which the eye at rest is unable to focus the image of a distant object on the retina.
Hyperopia, or far-sightedness, is an error of refraction in which, with the eye at rest, parallel rays from distant objects are brought to focus behind the retina. This can be caused by a relatively flat corneal surface and a concommitant decrease in the angle of refraction of rays as they pass through the refractive media of the cornea, causing a convergence or focus of the rays at a point behind the retina.
Myopia, or near-sightedness, is that refractive condition in which parallel rays are brought to focus in front of the retina. One condition that commonly causes myopia is an excessively steep corneal curvature. Thus, the angular refraction of rays is greater as they pass through the refractive media of the cornea, and the over-refracted rays converge or focus in front of the retina in the vitreous of the eye. When the rays reach the retina they become divergent, causing a blurred image.
Astigmatism is a condition in which the front surface of the cornea is not truly spherical. Although the eye may be perfectly healthy, the corneal surface may be toroidal in nature as a result of orthalyonal meridians having discrepancies in curvature. This causes the horizontal and vertical meridians to focus apart from one another. Some degree of astigmatism is normal; however, more severe astigmatism causes the blurring of lines aligned at a particular angle. A person with astigmatism may see horizontal lines clearly, but vertical lines blurred, or the blurring may occur in an oblique meridian.
The normal treatment of refractive error involves the use of eyeglasses or contact lenses, both of which have well-known disadvantages. Consequently, recent research has been directed to surgical procedures to change the refractive condition of the eye. Such procedures are generally referred to as "kerato-refractive techniques," particular examples of which are radial keratotomy, keratomileusis, keratomileusis-in-situ, epikeratophakia, keratophakia, photo-refractive keratectomy, and mechanical keratectomy.
In radial keratotomy the cornea is reshaped to correct myopia through the use of a plurality of incisions made to extend radially out from the central axis of the eye. The associated scarring essentially stress-relieves the cornea, causing the cornea to become flatter and less powerful in the central regions due to the outward force on the periphery by the eye's intraocular pressure.
Keratomileusis involves the regrinding of a corneal lamella into a meniscus to correct myopia or hyperopia. In this procedure, a portion of the cornea is excised (keratectomy) in the shape of a disk. The corneal disk is then frozen and its shape is altered by the use of a lathe. After lathing, the disk is thawed and sutured onto the keratectomy bed. In keratomileusis-in-situ the underlying stroma is re-shaped to correct the refractive error present in the eye.
Keratophakia involves removing corneal tissue and sandwiching another material therebetween. Typically, a homograft is ground into a convex lens which is placed interlamellarly to correct aphakic hypermetropia. In epikeratophakia a corneal tissue applique is attached to the corneal surface to correct the refractive error such that the patient, in effect, receives a "living" contact lens.
Photo-refractive keratectomy, also known as photoablation, and corneal contouring are techniques that correct refractive error by modifying the contour of the cornea through tissue removal. In photoablation a laser is used to reshape the surface of the cornea by removing intra-stromal tissue. Corneal contouring, on the other hand, is a mechanical keratectomy procedure that re-profiles the cornea by rotating and oscillating a knife edge about the optical axis of the cornea, scraping the cornea until the refractive error has been substantially corrected.
A number of these techniques, derivatives and improvements thereof, and instruments used in connection therewith are described in further detail in various medical resource publications and issued patents. For example, U.S. Pat. Nos. 4,526,171; 4,619,259; 4,688,570; 4,691,715; 4,691,716; and 4,665,914 relate to radial keratotomy, while U.S. Pat. Nos. 4,662,370; 4,432,728; 2,249,906; 2,480,737, 4,750,491; and 4,763,651 pertain to corneal trephines. Laser or electrical surgical procedures and instruments are disclosed in U.S. Pat. Nos. 4,665,913; 4,718,418; 4,729,372; 4,770,172; 4,798,204; 4,732,148; 4,724,522; 4,838,266; 4,840,175; 4,381,007; 4,326,529; 4,994,058; 4,988,348; 4,973,330; and 4,941,093. U.S. Pat. Nos. 4,947,871; 4,997,437; 4,173,980; and 4,834,748 concern techniques and devices for grinding or abrading the corneal surface, and mechanical keratectomy using a scraping mechanism is revealed in U.S. Pat. No. 5,063,942. Other methods of surgical manipulation of the cornea are disclosed in U.S. Pat. Nos. 4,907,585; 4,907,587; 4,766,895; 4,452,235; 4,671,276; 4,976,719; 4,712,543; and 4,461,294.
An excellent resource on corneal surgery in general is Microsurgery of the Cornea. J. Barraquer and J. Rutllan (1984) Ediciones Scriba, S. A.--Barcelona.
Although the above-identified surgical procedures have a broad application in the correction of hyperopic, myopic, and astigmatic errors, there are significant obstacles yet to be overcome. For instance, keratophakia, keratomileusis and epikeratophakia are expensive, complicated procedures, limited in success by scarring in the corneal stromal interfaces. And eyes having undergone keratotomy may similarly suffer from post-operative instability, a significant decrease in impact resistance, scarring, glare, and vision fluctuation. Precise corneal reshaping is further hindered due to individual variances in wound healing, the resiliency of the cornea, the effect of fluid pressure, and the skill of the surgeon in applying the reshaping tool.
All of this leads to perhaps the most significant problem associated with kerato-refractive surgery--unpredictability of result. In fact, the incidence of over-or under-correction associated with these surgeries serves to dissuade many suitable candidates from undergoing a surgical refractive correction. In addition, errors in correction of refractive shift (hereinafter sometimes referred to as "corrective error"), especially radial keratotomy hyper-corrections, many times necessitates a second surgical procedure to rectify the over or under correction. The problem is no different for laser surgeries. Intra-stromal ablation using infrared wavelengths has not yet reportedly yielded the precision necessary to achieve predictable postoperative results, and surface ablation with an excimer laser is reported to result in unpredictability attributable to high degrees of variability of ablation depths per laser pulse. Thus, it is not possible to precisely pre-calculate a procedure to achieve a defined post-operative result.
Owing to the difficulties in calculating a proper surgical protocol and predicting kerato-refractive surgical results to ensure accurate and precise correction, there is a need for a method and apparatus to perform visual acuity testing, utilizing real-time patient responses to visual prompts, during and immediately after ocular surgery.
Presently, the patient's corneal topography, refraction and visual acuity are determined prior to the initiation of the procedure. Based upon this pre-operative testing, the surgical protocol for the corrective procedure is calculated using given parameters. While the patient's changed corneal topography may be evaluated during the operation, only long after the completion of the procedure, is the patient's "corrected" visual acuity subject to evaluation.
As it stands now, meaningful evaluation of a patient's corrected visual acuity is delayed to between two and four weeks after the time of surgery, and even longer in the case of radial keratotomy. The lack of a method and device to determine a patient's visual acuity during and immediately after the corrective procedure deprives the surgeon of the ability to accurately adjust the surgical protocol during the procedure to take into account differences in surgical performance or individual patient variables such as corneal density and the degree of corneal hydration, variables which may be concealed in the pre-operative stage. The availability of a method and apparatus to measure the patient's visual acuity during and immediately after an ocular surgical procedure would therefore increase surgical precision and accuracy of result by allowing the surgeon to make on-the-spot adjustments to standard surgical protocol in response to patient cues. This should decrease both the instance of over and under correction associated with kerato-refractive surgery and the resultant necessity for second surgeries.
Though the above discussion has focused on refractive errors and keratorefractive surgical corrections, it should be understood that the method and apparatus disclosed and claimed herein have utility under any conditions in which patient responses to visual prompts during or immediately after ocular surgery are desirable.